This invention relates to the measurement of stress in a fastener under tension utilizing ultrasonic signals.
There are many methods for measuring tension in a bolt or other fastener. Perhaps the most commonly known is the use of a torque wrench which measures the torque applied as the bolt is installed. This applied torque is roughly proportional to the tension. Ultrasonic signals can be used to give a more accurate measure of tension in a bolt while the bolt is being installed. U.S. Pat. No. 3,969,810 to Pagano shows a method for measuring the tension in a bolt by measuring the time-of-flight for an ultrasonic signal which is transmitted down the length of the bolt and reflected back. This time-of-flight is first measured when the bolt is not under stress and is then measured as the bolt is placed under stress, with the variation in the time-of-flight being approximately proportional to the tension.
U.S. Pat. No. 4,014,208 to Moore discloses another method for measuring tension in a bolt while it is being installed using ultrasonic signals. A pair of pulses are transmitted down a bolt before installation, and the frequency at which the second echo of the first pulse coincides with the first echo of the second pulse is noted. As the bolt is installed, the frequency is varied to maintain the coincidence of these echoes. The change in frequency is then used to calculate the tension.
It is desirable in some circumstances to measure the tension in a bolt that has already been installed without loosening and retightening the bolt. Such a method is disclosed in U.S. Pat. No. 3,975,948 to Makino. This patent shows a method for determining tensile stress in a bolt which has already been installed by using longitudinal and transverse ultrasonic waves. A longitudinal wave can be visualized as the wave produced by hitting the end of the bolt with a hammer. A transverse wave can be visualized by thinking of the bolt as a clothesline with a wave being sent down its length by moving one end rapidly up and down. A capacitor in a transmitting circuit is adjusted so that first the frequency of the longitudinal waves, and then the frequency of the transverse waves, become one of the resonant frequencies of the bolt. The tensile stress of the bolt is then calculated using the determined resonant frequencies for the longitudinal and transverse waves in accordance with a mathematical formula. The formula requires the use of a calibration curve which must be separately determined for each bolt type and each different configuration of the stressed and unstressed portions of the bolt.
Makino thus discloses a method for measuring the tension in a bolt or other fastener which is already under stress, but this is accomplished at the sacrifice of simplicity by requiring adjustments to a capacitor to determine resonant frequencies.